A semiconductor package has a resin-encapsulation portion to protect a semiconductor element. For formation of the resin-encapsulation portion (for encapsulation of a semiconductor element), a curable resin e.g. a thermosetting resin such as an epoxy resin is used.
As a method for producing a semiconductor package, for example, a method comprising an encapsulating step by a so-called compression molding method or transfer molding method is known, wherein a substrate having a semiconductor element mounted thereon, is disposed so that the semiconductor element is located at a predetermined position in the cavity of a mold, and a curable resin is filled in the cavity to form a resin-encapsulation portion. In such a method, in order to improve releasability by preventing adhesion of the encapsulation resin and the mold in the encapsulation step, such a measure has been adopted that a mold release agent is blended to the curable resin, or a mold release film is disposed on the cavity surface of the mold.
In the step of encapsulating the semiconductor element, even if a mold release film is disposed, there is a problem such that a gas generated from the curable resin (outgas) or a low-viscosity substance is transmitted through the mold release film and gets in contact with a high temperature mold to contaminate the mold. Further, the disposition of a mold release film on the cavity surface of the mold is carried out by letting the mold release film be adsorbed and held to the cavity surface by vacuuming, whereby volatile components such as oligomers in the mold release film are likely to migrate to the mold side to cause mold contamination. If mold contamination occurs, it is inevitable to pause the step of encapsulating the semiconductor element in order to clean the mold, whereby the production efficiency for the semiconductor package is reduced.
To overcome such problems, the following mold release films (1) and (2) have been proposed (Patent Documents 1 and 2).
(1) a gas barrier semiconductor resin mold release film comprising a release layer excellent in releasability and a plastic support layer for supporting it, wherein the strength at 200% elongation at 170° C., of the plastic support layer is from 1 to 50 MPa, and the xylene gas permeability at 170° C. is at most 5×10−15 (kmol·m/(s·m2·kPa)).
(2) a gas barrier semiconductor resin mold release film comprising a release layer excellent in releasability, a plastic support layer for supporting it, and a gas permeation inhibiting layer made of a metal or a metal oxide, which is formed between the release layer and the support layer, wherein the xylene gas permeability at 170° C. is at most 5×10−15 (kmol·m/(s·m2·kPa)).
The mold release films (1) and (2) are said to be able to reduce mold contamination due to outgassing and low viscosity material, since the xylene gas permeability is at most 5×10−15 (kmol·m/(s·m2·Pa)).
In the mold release films (1) and (2), a release layer is provided at least on the surface to be in contact with a mold resin injected into the mold, in order to impart sufficient releasability to the mold resin (resin-encapsulation portion) after curing. As such a release layer, a layer made of a fluororesin is used.
Further, in the production of a semiconductor package, in order to display information of the product number, manufacturers, etc., it is common to form an ink layer on the surface of the formed resin-encapsulation portion by printing with an ink.
However, if a release agent is blended to the curable resin, since the release agent will be present at the surface of the resin-encapsulation portion to be formed, adhesion between the resin-encapsulation portion and the ink layer tends to be low, whereby there will be a problem such that with time, the ink layer tends to be peeled off from the resin-encapsulation portion. Therefore, it is necessary to prevent the reduction in adhesion by applying some treatment (for example, Patent Document 3), which leads to an increase of process steps.
In a case where without blending a release agent to the curable resin, a mold release film is disposed on the cavity surface of a mold, no problem due to a release agent arises, but the adhesion between the resin-encapsulation portion and the ink layer may still not be sufficient.
In order to improve the adhesion between the resin-encapsulation portion and the ink layer, it has been proposed to use a mold release film having its surface roughness increased by forming irregularities on the surface, and to form a resin-encapsulation portion by disposing the mold release film in a mold so that the irregularities face the curable resin side (for example, Patent Document 4). In this case, the irregularities of the mold release film surface are transferred to the surface of the resin-encapsulation portion. By the presence of such irregularities, the adhesion of the ink layer to the resin-encapsulation portion will be improved.
The mold release film is required to have, in addition to releasability, heat resistance to withstand the temperature of the mold during molding, strength to withstand the pressing force or the flow of the encapsulation resin, etc. It has been proposed to use a fluororesin film as a mold release film, since it is excellent in these properties (for example, Patent Document 5).